Electrospinning technology has been developed rapidly in the last decade. The fabricated submicron-size diameter fibers formed from natural and synthetic polymers have been applied widely to different research areas and industries, such as tissue engineering, drug delivery, sensors and electrodes for use in electronics, gas storage, and air purification. During the electrospinning process, when the applied voltage is strong enough to overcome the surface tension of the (charged) polymer solution at the tip of the spinneret, a fine jet stream is ejected. As this jet stream travels from the spinneret to the collector, the solvent in the jet stream is evaporated while the polymer solution begins to form a thinner and thinner jet stream, resulting in the formation of a non-woven fibrous scaffold at the collector. The fiber diameter and the morphology are affected by both processing and materials parameters. A relatively new utilization of the electrospun nanofibrous scaffolds is their application in water purification.
Electrospun membranes have a high porosity (>80%) with fine diameters (from about 0.1 μm to about 1 μm) and an interconnected-pore structure, which yields a relatively very high specific surface area. As a typical demonstration, electrospun nanoscaffolds were used as a supporting layer in the thin-film nanofibrous composite (TFNC) ultra- and nano-filtration membranes, with the demonstrated permeation flux increased by a factor of from about 2 to about 10 when compared with typical commercial membranes. Another representative application of electrospun scaffolds (self supported or supported with a non-woven substrate, e.g., polyethylene terephthalate (PET)) that has been demonstrated in water purification applications is the use of such a microfilter for removal of nanoparticles which can be regarded as a model for waterborne bacteria, e.g., E. coli and B. diminuta, in drinking water. The pore size and pore size distribution of electrospun polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polysulfone (PSU), polyvinyl alcohol (PVA), and regenerated cellulose scaffolds have been investigated. For the removal of bacteria in contaminated drinking water, the pore size and pore size distribution of the electrospun membrane have to be improved in order to remove bacteria by size exclusion.
Typically, the diameter of waterborne bacteria is larger than about 0.2 μm. For instance, the dimension of B. diminuta is 0.3 μm×0.9 μm and that of E. coli is 0.5 μm×2.0 μm. Then, it becomes essential that the average pore size of the electrospun membrane should be less than 0.2 μm. Moreover, the pore size distribution must be relatively narrow in order to achieve a high retention value for bacteria (e.g., a 6 log reduction value—“LRV”). The average pose size of the electrospun membrane can be related to the average filter diameter at constant porosity. To approach a 0.3 μm average pore size, the average fiber diameter of the electrospun mat should be less than the 100 nm range. The nanofibrous composite membranes of electrospun PVA and PAN on PET support could approach this goal.
Another concern is that the surface of electrospun nanoscaffolds has to be functionalized so as to adsorb small viruses (or metal ions) which are not being removed by size exclusion but by adsorption (or complex formation). Typical modification methods include chemically grafting charged groups or ligands where many reaction steps would be involved, or by physical absorption of active groups where the attachments are often not durable. An alternative is the incorporation of functional additives into the electrospinning polymer solution; however, all electrospinning parameters would then be affected. Thus, the modifications require specific pathways for different types of functionalizations.
There are representative patents, e.g., U.S. Pat. No. 5,085,780 to Ostreicher, (Cuno, Inc., issued Feb. 4, 1992), WO 00/37385 to Wei (Kimberly-Clark Worldwide Inc., published Jun. 29, 2000), U.S. Patent Publication No. 2002/0155225 to Yeh et al. (Cummings & Lockwood, published Oct. 24, 2002), and U.S. Pat. No. 6,565,749 to Hou et al. (The Procter & Gamble Company, issued May 20, 2003), that have provided a facile method in employing charges on fibrous filters (e.g., glass fibers) by surface coating with positively charged polymers (e.g., polyamino-polyamide, polyethylenimine (PEI)) and followed by cross-linking with epichlorohydrin or multi-epoxy groups. All of the patents and patent publications cited in this paragraph are incorporated herein in their entirety.
In U.S. Pat. No. 5,085,780 to Ostreicher, cellulose fibers/silica-based particulates/fiber-filter elements were used as the substrate, whereby the surface of the fibers was coated with polyamino-amine-epichlorohydrin/polyamine-epichlorohydrin resin as the primary layer. In order to improve the capacity of the filter, a secondary amine (e.g., ethylenediamine) was also used for further modification. The filters were then challenge tested with Metanil yellow (4 ppm), 0.1 μm latex particle suspension, and Pyrogen aqueous solution (with the pH ranging from 7 to 12) for the adsorption process.
WO 00/37385 to Wei uses similar modifications but based on glass fibers/cellulose fibers as the support. Moreover, acrylic resin binder was used in the polyamino-amine-epichlorohydrin/polyamine-epichlorohydrin resin system.
U.S. Patent Pub. No. 2002/0155225 to Yeh et al. modified polyester (e.g., polyethylene terephthalate (PET)) by hydrolyzed with NaOH or aminolyzed with NH3 (aq.) as the substrate. After hydrolyzation, a carboxylate group/amide group was produced that further reacted with epoxy-ammonium reagents. After modification, the ammonium group was introduced on the surface of the polyester filter which provided the adsorption capacity of Metanil yellow (10 ppm) at pH 9. However, only static adsorption experiments were performed in this patent.
U.S. Pat. No. 6,565,749 to Hou et al. employed mainly different grades of glass fibers from Ahlstrom Corporation, Helsinki Finland, as the substrate and modified polymers containing quaternary ammonium units, PEI cross-linked with diglycidylether of 1,4-butanediol, and polyamide prepared by condensation of poly(methyl bis(3-amino propylamine)) with dicarboxylic acid. The adsorption capacity of those filters was comprehensively challenge tested with both bacteria (including Klebsiella terrigena, E. coli, and B. diminuta) and a bacteriaphage MS2. The filter could withstand up to 5000 mL of bacteria or viruses (concentration from about 106 to about 108 pfu/mL) suspension before failure and up to 1000 mL of the suspension before LRV of the filter started to decrease.
Microfiltration membranes have been used in various applications including cell storage and delivery systems as disclosed in U.S. Pat. No. 6,790,455 to Chu et al., which is incorporated herein in its entirety.
Extending the usage life of microfiltration filters is a major challenge faced by all commercial microfiltration membranes. Polyethylenimine (PEI) has been shown to be an effective adsorbant for viruses. However, without effective immobilization, it can be washed out quickly. A PAN/PET filter (e.g., AWA-16-1; referred to hereinafter as “AWA” is available from Sanko Ltd. Co., Japan) modified with PEI can have 4 log reduction value (LRV) against bacteriaphage MS2 but only for short time periods. PEI can be stabilized in PAN/AWA by cross-linking. The di-epoxy group is a good candidate to cross-link PEI with a high degree of cross-linking and controlled reaction rate. In addition, glutaraldehyde (GA) is another option for the cross-linking reaction, where a high flux, high retention, and long life (high capacity) nanofibrous microfiltration filter can be achieved for water purification applications.